In aviation, such actuators are used at present for flight controls, and they can have non-negligible advantages over hydraulic actuators.
When centralized electricity generation is provided, the advantage of such actuators can be seen even more clearly, particularly if it is desired to use them for driving landing gear legs.
The driving element of the screw and nut system may be the screw or the nut, however it is often preferred for the system to have the screw as the driving element and the nut as the driven element, with the screw being prevented from moving axially and being constrained to rotate by means of a coaxial gear wheel that meshes with the outlet gear wheel of a motor and gear box assembly, while the nut is secured to a guide rod and sliding in the main body. Such a solution provides good protection against shock and against external pollution (because of the guide rod), better stability against buckling in the extended position (because the guide rod is guided in the main body), and better capacity for greasing the screw. Furthermore, the installation of anti-rotation systems for the guide rod and for its end-of-stroke abutments is facilitated: as a result, in particular when driving an aircraft landing gear leg, the above-described configuration appears to be more suitable than the opposite configuration.
Nevertheless, that type of linear actuator suffers from drawbacks that may be severe if a major mechanical failure should occur (a tooth breaking, seizing where the teeth engage and/or in the bearings): this is most particularly the case when the actuator is associated with driving an aircraft landing gear leg, since a fault of the above type can cause the screw of the screw-and-nut system to jam before the landing gear has been lowered or while it is being lowered, and the inertia of the landing gear and the aerodynamic effects exerted thereon can then insufficient to unjam the actuator.
Although the probability of such a fault is low (in aviation, it is always desirable to achieve a failure rate of less than 10.sup.-9 per hour), the consequences that stem therefrom are nevertheless very severe.
That is precisely the problem which the invention seeks to resolve, even though its field of application is not strictly limited to that of actuators for driving landing gear legs.